Heterochrony in human evolution: The case for neoteny reconsidered

This paper reviews the evidence for the claim that neoteny , or morphological juvenilization resulting from dissociation and retardation of ancestral rates of shape change, has played a key role in human evolution. Accepted categories and processes of heterochrony are reviewed, and the available data on human growth, variation, and evolution are analyzed in light of expected results. Relatively weak concordance with predictions is found. Mistakes of fact and interpretation fall into several categories, including (1) confusion of neoteny with paedomorphosis resulting from other heterochronic processes; (2) conflation of growth prolongation in time with morphological shape retardation; (3) failure to move beyond superficial shape similarities to underlying homologous growth processes and patterns; (4) failure to identify a paedomorphic basis for key anatomical novelties in human evolution; and (5) establishing an essentially untestable framework for analysis of the hypothesis. Key areas that might contribute new data to this debate are discussed, particularly the genetic and epigenetic control of the covariation of morphology and development during ontogeny and evolution. The primary reasons that arguments in favor of neoteny in human evolution have persisted probably relate to anthropocentric factors and the search for a single basis for the important morphological and behavioral transformations characterizing our lineage.